Compressional and extensional tectonics in low-medium pressure granulites from the Larsemann Hills, east Antarctica

Abstract

<jats:title>Abstract</jats:title><jats:p>Meta-sediments in the Larsemann Hills that preserve a coherent stratigraphy, form a cover sequence deposited upon basement of mafic–felsic granulite. Their outcrop pattern defines a 10 kilometre wide east–west trending synclinal trough structure in which basement–cover contacts differ in the north and the south, suggesting tectonic interleaving during a prograde, D<jats:sub>1</jats:sub> thickening event. Subsequent conditions reached low-medium pressure granulite grade, and structures can be divided into two groups, D<jats:sub>2</jats:sub> and D<jats:sub>3</jats:sub>, each defined by a unique lineation direction and shear sense. D<jats:sub>2</jats:sub> structures which are associated with the dominant gneissic foliation in much of the Larsemann Hills, contain a moderately east-plunging lineation indicative of west-directed thrusting. D<jats:sub>2</jats:sub> comprises a colinear fold sequence that evolved from early intrafolial folds to late upright folds. D<jats:sub>3</jats:sub> structures are associated with a high-strain zone, to the south of the Larsemann Hills, where S<jats:sub>3</jats:sub> is the dominant gneissic layering and folds sequences resemble D<jats:sub>2</jats:sub> folding. Outside the D<jats:sub>3</jats:sub> high-strain zone occurs a low-strain D<jats:sub>3</jats:sub> window, preserving low-strain D<jats:sub>3</jats:sub> structures (minor shear bands and upright folds) that partly re-orient D<jats:sub>2</jats:sub> structures. All structures are truncated by a series of planar pegmatites and parallel D<jats:sub>4</jats:sub> mylonite zones, recording extensional dextral displacements.</jats:p><jats:p>D<jats:sub>2</jats:sub> assemblages include coexisting garnet–orthopyroxene pairs recording peak conditions of ∼ 7 kbar and ∼ 780°C. Subsequent retrograde decompression textures partly evolved during both D<jats:sub>2</jats:sub> and D<jats:sub>3</jats:sub> when conditions of ∼ 4–5 kbar and ∼ 750°C were attained. This is followed by D<jats:sub>4</jats:sub> shear zones which formed around 3 kbar and ∼ 550°C.</jats:p><jats:p>It is tempting to combine D<jats:sub>2–4</jats:sub> structures in one tectonic cycle involving prograde thrusting and thickening followed by retrograde extension and uplift. The available geochronological data, however, present a number of interpretations. For example, D<jats:sub>2</jats:sub> was possibly associated with a clockwise <jats:italic>P–T</jats:italic> path at medium pressures around ∼ 1000 Ma, by correlation with similar structures developed in the Rauer Group, whilst D<jats:sub>3</jats:sub> and D<jats:sub>4</jats:sub> events occurred in response to extension and heating at low pressures at ∼ 550 Ma, associated with the emplacement of numerous granitoid bodies. Thus, decompression textures typical for the Larsemann Hills granulites maybe the combined effect of two separate events.</jats:p>